1. Field of the Invention
The present invention relates to a video-on-demand storage server and, more particularly, to a method and apparatus for optimizing data retrieval performance in disk drives exploiting zone bit recording.
2. Description of the Related Art
Currently, video tapes may be rented at video rental stores. Typically, the video tapes are rented for one to three nights at a time. The subject matter of the available tapes include movies, entertainment games, and the like.
There are certain disadvantages in renting video tapes from video rental stores. First, the selection of movies at each video rental store is limited. Second, the number of rentable copies of each particular video tape is also limited. As a result, all the people seeking to rent a recently released popular movie will not be able to rent a video tape containing a copy of the movie. Hence, the availability of the video tapes cannot be guaranteed. Third, the people renting video tapes are required to go to the video rental store to select and rent a video tape, view the tape, and then return to the video rental store to return the tape. It is time consuming and typically inconvenient for the user to frequent the video store twice in order to view a video tape.
Lately, video-on-demand (VOD) systems have received a lot of attention because of their ability to solve these problems. In general, a VOD system would include a central storage server having numerous videos stored in digital form, a controller, a communication network, and customer equipment boxes. A customer would select an available movie (i.e., movie stored in the storage server) using the customer's equipment box. The controller would then transmit the selected movie over the communication network to the customer's equipment box. The customer then views the movie on his/her television without ever having to enter a video rental store.
Although file servers exist for applications such as on-line transaction processing, the architecture of the file servers is not well suited to VOD applications. As an example, on-line transaction processing tends to transfer numerous unrelated small pieces of data whose identities are disclosed only when urgently needed, whereas VOD servers are required to concurrently provide a large number of smooth streams of data. The rate of each stream is currently several times lower than the sustained transfer rate of a single magnetic disk drive. For example, a typical stream rate is between 1.5 and 6 Megabits per second, whereas the sustained transfer rate of a modern disk is between 15 and 30 Megabits per second.
There are also problems associated with existing VOD systems. In order to cost-effectively meet customer demand, video storage servers are required to provide a large number of current streams of data. Each storage server consists of one or more magnetic disk drives. These magnetic disk drives must be able to retrieve the data such that the streams of data can be smoothly transmitted at an appropriate speed for customer viewing. Because of the heavy demands placed on storage servers, there is a need for optimization of storage servers. In particular, there is a need to increase the number of streams that can be concurrently sourced by a storage server.
Classically, data is stored on magnetic disks such that the data transfer rate is independent of the track number. The number of streams that such disks can serve is largely independent of the reading location on the disks. This classical recording approach, however, does not make efficient use of magnetic disk recording area because the data transfer rate and rotation rate are fixed. As a result, the recording density is fixed by the inner most track. Hence, recording density is wasted in all other tracks having a larger circumference.
By using zone bit recording, the amount of data stored to a magnetic disk can be increased. That is, the recording density wasted is substantially reduced as compared with the classical case.
Zone bit recording is an approximation to fixed linear recording density. More particularly, with zone bit recording, a recording surface is divided into a sequence of concentric zones, and the number of sectors per track in any given zone is equal to the number that fit into the innermost track of that zone at the highest possible linear recording density. As an example, a disk might have between 7 and 15 zones.
A problem with using zone bit recording on a magnetic disk with a fixed rotation rate is that the transfer rate will vary considerably (almost by a factor of 2) with track location. As a result, even if access time can be neglected altogether, the number of streams that can be concurrently sourced depends strongly on the reading location. This problem does not occur in magnetic disk drives not employing zone bit recording. The problem also does not occur in compact disks because data, although recorded at a constant linear density, is read at a constant data transfer rate by varying the rotation speed.
Although one exploitation of zone bit recording would be to place more frequently viewed material in the outermost tracks, this simplistic approach is problematic because the frequency of viewing usage varies with time. Hence, constant rearrangement of data would be necessary. Such rearrangement of data is a task that not only burdens the video server but is otherwise not well understood.
Thus, there is a need for a video storage server that uses zone bit recording on its magnetic disks, yet achieves a substantially constant data transfer rate.